Biology Now, 2e

Toxic Plastic ■ 105

the plastic mouse cages: the water bottles were

leaking and the plastic cage walls were hazy.

She asked around and found out that, months

earlier, a substitute janitor had used detergent

with a high pH instead of the normal low-pH

detergent to clean the cages and bottles. With

some lab work, she identified the chemical

oozing from the corroded plastic. “It took essen-

tially one washing with the wrong detergent,”

says Hunt, “and once it was damaged, [the plas-

tic] started to leach bisphenol A.”

Bisphenol A, or BPA, was first synthesized

in 1891. A synthetic hormone that, like nonyl-

phenol, mimics estrogen, BPA was tested in the

G 2 checkpoint

G 1

G 2

M

S

G 1 checkpoint

Cell cycle

regulatory

proteins

Cell cycle pauses if cell size or

nutrient supply is inadequate,

or if DNA is damaged.

Cell cycle pauses if cell size or nutrient

supply is inadequate, DNA is damaged,

or DNA replication is incomplete.

Figure 6.6

The cell cycle must pass through

checkpoints to proceed

Just two of the known cell cycle checkpoints are

depicted in this diagram. Checkpoints also operate

in both the S phase and the M phase (mitosis).

Q1: What could happen if the cell’s

checkpoints are disabled?

Q2: What is the advantage of stopping the

cell cycle if the cell’s DNA is damaged?

Q3: Which part of the cell cycle may have

been influenced in Soto and Sonnenschein’s

breast tumor cell experiments?

1930s by clinicians seeking a hormone replace-

ment therapy for women who needed estrogen,

but it wasn’t as effective as other substitutes. In

the 1940s and ’50s, the chemical industry found

another use for BPA, as a chemical component

of a clear, strong plastic. Manufacturers began

to incorporate it into eyeglass lenses, water and

baby bottles, the linings of food and beverage

cans, and more. Unfortunately, as Hunt found

out, BPA doesn’t necessarily stay in those prod-

ucts. Not all the BPA used to make plastics gets

locked into chemical bonds, and what doesn’t get

locked into bonds can work free—especially if

the plastic is heated, such as when a baby bottle

is warmed, or if the plastic is exposed to a harsh

chemical, as Hunt’s mouse cages were. Because of

its prevalence in products and its ability to leach

out of them, BPA is one of the most common

chemicals we are exposed to in everyday life.

To confirm the hypothesis that BPA caused

the mouse egg abnormalities, Hunt’s team

re-created the original event. They intention-

ally damaged a set of new cages and put healthy

female mice in them. They also had the mice

drink from damaged water bottles. Later, when

they examined the eggs of these mice, they saw

the same toxic effects as before: 40 percent of

the eggs had abnormal chromosomes. The eggs

showed errors in meiosis.

Meiosis is a specialized type of cell division

that kicks off sexual reproduction, the process

by which genetic information from two individ-

uals is combined to produce offspring. Sexual

reproduction has two steps: cell division through

meiosis, and fertilization. BPA was affecting the

first of these steps—meiosis.

We learned earlier that mitosis produces

daughter cells with the same number of chro-

mosomes as the parent cell. These non–sex cells

are called somatic cells. In contrast, meiosis

produces gametes, daughter cells that have half

the chromosome count of the parent cell. The

differences between mitosis and meiosis are

the reason why the somatic cells of plants and

animals have twice as much genetic information

as their gametes have. The double set of genetic

information possessed by somatic cells is known

as the diploid set (represented by 2n), and the

single set possessed by gametes is called the

haploid set (represented by n).

Fer tilization, the fusion of two gametes,

results in a single cell called the zygote. The